Generally, in order to improve the service life of cutting tools, a hard ceramic thin film made of titanium carbide (TiC), titanium nitride (TiN), titanium carbon nitride (TICN) or alumina (Al2O3) is coated on the surface of a base metal for cemented carbide by chemical vapor deposition (CVD). Regarding cutting tools coated with Al2O3, a cutting tool comprising an Al2O3 thin film of 0.5-1 μm coated on a TiC thin film was introduced to the world for the first time in the world in 1973. The cutting tool comprising Al2O3 coated on TiC was slightly lower in toughness but significantly higher in wear resistance than a monolayer TiC thin film.
Also, in order to improve the toughness of cutting tools, a TiCN thin film is used which is coated using organic CN compound precursors (acetonitrile, CH3CN) at a temperature of 800-900° C. by middle temperature-chemical vapor deposition (MT-CVD). The deposition of the conventional TiCN thin film by high temperature-chemical vapor deposition (HT-CVD) was performed using gas materials including TiCl4, CH4, N2 and H2, at a temperature of about 1,000-1,200° C., whereas the coating by MT-CVD was performed using TiCl4, CH3CN, N2 and H2, at a temperature of 800-900° C. The TiCN thin film coated by MT-CVD is slightly lower in hardness than the TiC thin film, but it has hardness sufficient to provide an improvement in wear resistance and is excellent in toughness since it has a columnar structure.
In the case of Al2O3 thin films with excellent oxidation resistance, since it was reported in studies on phase control technology for Al2O3 thin films in the 1980s that alpha-alumina (α-Al2O3) and kappa-alumina (κ-Al2O3) thin films are suitable for cast iron and steel, respectively, control technology for the Al2O3 thin films has been rapidly developed and commercially used. The α-Al2O3 thin film among the Al2O3 thin films is a unique stable phase which does not undergo a phase change during processing, and it has the best hardness. Thus, it shows excellent cutting performance in cast iron processing which is performed under high-speed cutting conditions. It was analyzed that the κ-Al2O3 thin film is lower in thermal conductivity than the α-Al2O3 thin film and thus shows excellent wear resistance in steel cutting, which generates much heat.
In currently commercially available cutting tools, since the Al2O3 thin film is always applied to ceramic substrates, TiC or TiCN-coated carbides, the interfacial adhesion between the TiC or TiCN thin film surface and the Al2O3 coating layer is very important. In this regard, the TiC or TiCN thin film may be represented by the formula TiAlvBwCxNyOz (v+w+x+y+z=1, v, w, x, y, z≧0) wherein Ti in the TiC or TiN thin film is completely or partially substituted with Al, and carbon (C) or carbon nitride (CN) is completely or partially substituted with boron, oxygen or nitrogen.
EP Nos. 408535 and 603144 disclose coating the α-Al2O3 thin film on cutting tools in order to increase the wear resistance of the cutting tools in cast iron turning. However, such patents do not disclose the crystallographic structure of the α-Al2O3 thin film which shows excellent cutting performance.